Two-stage air swept ball mill



y 1950 H. E. LONNGREN 2,507,917

TWO-STAGE AIR SWEPT BALL MILL Filed May 12, 1948 INVENTOR.

Patented May 16, 1950 NHTED STATES orrica My invention relates topulverization of coal and like material aid has particular reference tothe art comminution with the aid of loosely arranged metal balls in ahorizontal rotating drum One object of my invention is to carry out thecomminution of coal and like material in two distinctly separate stepsin one and the same rotating drum with the aid of loosely arranged metalballs the first step of which comminution process is devoted to impactaction by the metal balls and the second step to attrition. This isaccomplished by partitioning of the interior of the drum into twodistinct compartments in which separate ball charges are used andactuated in such a manner that impact will result in one compartment andattrition in the other. I have found that the reduction in size by meansof metal balls is greatly expedited by imparting a great inertia to eachball. It is well known that the impact is a function oi the inertia.Crushing of the incoming coal requires impact whereas grinding to apowder depends upon friction between the balls. It is thereforeadvantageous to separate the two comminution steps entirely so as toobtain the best results from each action by the balls. In this mannerthe metal balls will last much longer because attrition does not takeplace in the entrance zone where the incoming coal particle size is suchthat attrition is of no avail.

Another object of my invention is to effect acceleration of the metalballs and the conversion of the imparted kinetic energy into usefulcomminution work in a controllable manner in each comminution step. Forthis purpose I provide means within the rotating drum whereby every ballis forced to participate in the process in like manner, that is, topound the coal in the first comminution step and to grind it in thesecond step. In both steps the balls are accelerated in like manner bybeing caused to travel momentarily with the peripheral velocity of therotating drum. In the first step every ball is free to continue with thegiven velocity until it strikes the'coal mass from above whereas in thesecond step its kinetic energy is given up to a mass of balls in itspath which is thereby stirred.

Still another object of my invention is to eiiect reduction in size ofcoal and like material in such a manner that an eventual wear of themetal balls or of the drum lining can not hamper the comminution steps.This is accomplished by providing a perfectly smooth lining against.such ribs are used in my apparatus.

erator member which is pivoted so as to adjust itself as wear sets in.

A further object of my invention is to provide a pulverizing apparatusof the horizontal rotating drum type in which a high coal level (abovethe centerline of the drum) can not hamper the ability of the apparatusto deliver an acceptable product. This is accomplished by providing adischarge opening which is positioned high above the centerline of thedrum so as to prevent the coal mass from clogging it.

A still further object of my invention is to expend the least possibleamount of energy -for the comminution of the material at hand. This isaccomplished by actuating the balls in such a manner that theirresistance to acceleration is the least, that is, by permitting them toroll between two smooth surfaces while kinetic energy is imparted tothem.

A specific object of my invention is to eifect withdrawal of groundmaterial from the pulverizing apparatus by means of a mass-flow of airso directed and disposed within the drum proper that a high velocity maybe used without withdrawing particles of an undesirable size. For thispurpose I introduce carrying air in a zone high above the coal level inthe first comminution zone thru which it is permitted to pass in thelongitudinal direction at a high velocity whereupon it is caused toenter the second comminution zone in the centripetal direction and toleave this zone in the centrifugal direction. This reversal of the flowdirection insures a proper precipitation of coarse particles in thesecond comminution zone. Only finely ground particles can be acceleratedin the centrifugal direction because the gravitational acceleration mustfirst be overcome.

Insofar as I am aware no prior disclosure of horizontal rotating ballmills anticipates the real substance of my invention which, as stated,1'89 lates primarily to a method of effecting comminution of coal andlike material in two distinctly separate comminution zones within therealm of one and the same rotating ball mill by imparting kinetic energyto each ball in like manner in the two zones and by converting thisenergy into full impact in one zone and to attrition in the other zone.Present types of horizontal rotating ball mills require longitudinalribs or ridges which actuate the ball charge. No Present types ofmachines cannot function properly when the coal level is above thecenter line of the ma- Whieh the balls are pressed by a smoothaccelchine because the withdrawal opening is thereby aco-1,01:

reduced and mass-ilowcarries too coarse particles out from the machine.This limitation is not present in my apparatus because thedischargepassageway is positioned high above the center line. The starting torqueis also high in present machines of the'rotating drum type because theentire ball charge must be actuated at.

to the acceleration. In present machines of the type in question it isin addition necessary to introduce carrying or floating air togetherwith the coal feed because a large flow area is required in order tokeep the velocity down. This in turn necessitates the use of large andcostly support bearings. In my machine this condition does not exist andthe air and coal are introduced sepflately with the result that smallerbearings serve the purpose. These bearings do not support the entiredrum and internal charge load because I transmit a considerable portionof the charge load to an internal structure-hereinafter fullydescribedwhich -is separately supported bymeans of stationaryplategirders.

My invention is fully disclosed in this specification and claimsreference being had to the accompanying drawing.

In the accompanying drawing Fig. I represents a sectional elevation ofan apparatus constructed in accordance with my invention and comprisesessentially an elongated horizontal drum or cylinder mounted withtrunnions in support bearings in a rotatable manner, driving mechanismfor the drum, a specially shaped internal structure which divides theinterior of the drum into two compartments or comminution zones, supportgirders for the internal structure, loosely arranged metal ball chargesin each compartment in the drum, specially shaped accelerator memberspivotaliy mounted on the internal structure, means to feed coal into theapparatus, means to introduce carrying air into the apparatusand meansto withdraw a final product from the apparatus.

'- l'ig. II represents a cross-sectional view taken along line II-JI inFig. I.

Fig. III represents a cross-sectional view taken along line III-III inFig. I.

These features and others will more fully and at large appear from thefollowing detailed description.

"Referring in detail to Fig. I, a feed of precrushed coal is suppliedthru hopper I at the bottom of which a belt conveyor 4 picks it up andtransports it into the apparatus proper wherein it first is subjected toa crushing to minute particles in a crushing zone 6 and to pulverizationto afine powder in a pulverization zone I. Amotor I with conventionalgears and drive shafts operates the belt conveyor. The comminution iseffected by means of a multiplicity of round metal balls I in each zoneas each ball therein is caused to travel with a velocity equal to theperipheral velocity of the rotating drum 9 and the imparted kineticenergy is converted into useful comminution work. In the crushing zonethe imparted kinetic energy of each ball is converted into imiii ballsin its path. From that in the crushing zone continues to travel freelywith the imparted velocityuntilitstrikesthecoalbedinitspath. whereas inthe pulverization zone as shown in I'll. III each accelerated ball losesits inertia upon c0ntactingtheballmassinitspath. Thetwolones I and I areformed by a stationary partitioning plate II which is held in place byan internal stationary structure which. comprises an air inlet portionll, shafts l2, II, a withdrawal portion l3 and tie bars It. The entirestructure is supported independently at the end sleeves II and It bysupport plate girders i1, I! in such a manner that the drum I can freelyrotate around said structure. Sleeves I I and II are mountedconcentrically'with drum trunnions It and it so as to permit freerotation in bearings II. A conventional motor driven gear drive 2|rotates the drum. Leakage is prevented by packing glands 22, 22. Theinner envelope of the drum shell is provided with a renewable wearresistant lining 23 the inner surface of which is perfectly smooth.Acceleration of the metal balls is effected by means of specialaccelerator members 24, 24 pivotally mounted on shafts i2, I: in such amanner that their dead weight furnishes sui'iicient grip or friction tocause each ball to travel with the rotating drum. Minute particles ofcoal are conveyed thru the apparatus in entrainment with air which isintroduced thru a multiplicity of apertures 28 in end portion ll of thestationary internal structure. A segmental passageway It connects thecrushing zone with the pulveriza tion zone. Baiile plate 21 in thepulverization zone directs the mass-flow in the centripetal directionwhich causes minute particles to precipitate readily. The final groundproduct is carried away by the air mass-flow which sweeps around thebaiiie plate 21 in the centrifugal direc-' tion in the pulverizationzone. Withdrawal from the pulverization zone takes place thru asegmental passageway 28 which communicates with conduit 29 in thewithdrawal portion II. Sleeve ll serves also as outlet conduit and ispreferably equipped with an upward turned elbow piece onto which thedischarge pipe II is bolted. Sleeve It is provided with a partitionplate ll so as to form a separate carrying air passageway 32 whichcommunicates with conduit 33 in the air inlet portion i I. Carrying airis supplied from a plenum box M.

It is pointed out that the amount and size of the balls in thecomminution zones is determined on the basis of the duty to be performedtherein by each ball. In the first zone-the crushing zone-the magnitudeof the impact by each ball is the governing factor. In this zone Itherefore use large balls-from about 3 to 4 inches in diameter-becausethe kinetic energy is a fimction of the mass. The amount of balls isabout 50 to percent in excess of the amount which it takes to cover theprojected area of the accelerator member in this zone. In the secondsanepulverization zone-the comminution work is based upon attrition. Alarge surface area of the ball mass is therefore a prerequisite. Forthis reason I use smaller balls in this zone-from about 1 to 2.5 inchesin diameterin an amount equal to about 5 to 20 times the amount which isrequired to cover the projected area of the accelerator member. a

A readily crushable material-such as soft coal-requires less expendedenergy for comminution than a harder material, as for instanceanthracite coal. With present types of machines t is necessary to employa large amount of balls. '.hat is, larger machines are required when thematerial is harder. This is so because their funcdoning depends upon theweight of the ball :harge for crushing purpose. With my apparatus it isfeasible to pulverize materials of varyng hardness simply by varying therotational weed of the drum. As this speed is increased additionalkinetic energy is imparted to the balls with the result that more energycan be con- ;erted into comminution work. While the roaational speed ofpresent types of machines :annot be much higher than about 25 R. P. M.[f excessive wear is to be avoided-my machine in the other hand may wellbe operated at speeds 1p to about 100 R. P. M. without encounteringJrohibitive wear. This is readily appreciated from the fact that anincreased speed does not :ause a material increase in wear when eachball :an roll freely while it is being accelerated. The 5 ncrease inkinetic energy varies as the square if the change in velocity.

Moreover, in present types of horizontal rotat- .ng ball mills it isnecessary to create and main- :ain a definite eccentricity of the ballcharge in .he machine because only when the ball charge .evel is at acertain slope with the horizontal plane is it possible to eifect aproper absorption if the coal feed by the ball charge. In other words, aconsiderable portion of the power input 5 consumed tocause thiseccentricity. This re- ;uirement results inevitably in rapid wear of theactuating drum lining and the balls themselves and it is not uncommonthat the degree of lineiess actually becomes greater than desirableiimply because the required number of balls is iiltated by the necessarydead weight to accomallsh crushing of the coal feed down to a size whenactual grinding can take place rather than w the attrition requirementsfor the grinding alone. In my apparatus these conditions and re-:uirements are obviated. The accelerator memaers in my machine aremounted in a sloping )OSltlOll which causes the balls and the coal feedto mix properly and to always find the accelera- :ion zone in themachine.

' It will be understood that numerous modifications of the invention maybe made without departing from the spirit thereof, and the saidinvention is not limited to the precise details herelnbefore enumeratedbut is to be construed as broadly as the appended claims will permit.

What I claim is:

A two stage air swept ball mill comprising an elongated rotatablymounted horizontal drum having a renewable smooth shell liner, anexternally supported stationary structure within said 6 drum adapted toprovide stationary conduits tor the material and air at its end portionsand to partition the internal drum space into a crushing zone and agrinding zone the partitioning member of which structure is of lessheight than the space within the drum so as to provide a segmentalpassageway between said zones thereabove, metal balls of identical sizeloosely arranged in said crushing zone so as to travel therein freely,metal balls of identical size loosely arranged in said grinding zone soas to be stirred therein, an accelerator member having a convex lowerface pivotally supported on said internal structure at the base thereofin each zone so as to provide an annular space between the shell linerthrough which space only one layer of metal balls may pass at one andthe same time said accelerator member being adapted to furnish apressure upon said metal balls in said annular space whereby theresulting flriction causes the rotary motion of the drum to acceleratesaid balls and to rotate them about their own axis, means in the feedend conduit of said stationary structure to independently feed materialinto the crushing zone at the axis of the drum, means in the feed andconduit of said stationary structure to independently conduct air intothe crushing zone at an elevation high above the axis of the drum in theform of a plurality of horizontally directed Jets, 9. segmental baiilein the grinding zone adjacent to the partitioning member with an upperarcuate edge closely adjacent the liner and a lower edge below the topof the partitioning member and means to withdraw a final product inentrainment with air from the grinding zone at an elevation high abovethe axis of the drum.

HARALD E. LONNGREN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date Re. 15,930 Sherban Oct. 7, 1924926,441 Shafter June 29, 1909 1,491,841 Bell Apr. 29, 1924 1,628,609Newhouse May 10, 1927 1,690,712 Bonnot Nov. 6, 1928 1,788,825 Danks Jan.13. 1931 FOREIGN PATENTS Number Country Date 4,899 Great Britain Mar. 7,1895 296,697 Great Britain Sept. 3, 1928

